The interactions of short pulse lasers with matter are interesting subjects not only in applications such as surface fabrication but also in physical phenomena for study. Optical short pulse lasers have abilities to occur the ablation phenomena accompanying the creation of high temperature, high pressure, and excited states of electrons. The picosecond soft x-ray laser (SXRL) pulse also has ability to occur the ablation. The SXRL having the wavelength of 13.9 nm and duration of 7 ps is one of attractive x-ray source for ablation study, because the ablation threshold obtained with the focused SXRL pulse is much smaller than those obtained with other lasers having longer durations and/or longer wavelengths. The low ablation threshold of a material for the SXRL beam has a possibility of efficient nanometer scale surface machining by an ablation. The ablation study will lead to the physical research and the direct surface machining. In addition, the wavelength of the SXRL is very close to the wavelength of the extreme ultraviolet (EUV) lithography system (λ = 13.5 nm). In the presentation, we report on development of the soft x-ray laser irradiation system. The irradiation system has an intensity monitor based on the Mo/Si multilayer beam splitter. This intensity monitor provides the irradiation energy onto sample surface. The SXRL has an ability to confirm the ablation threshold and to examine the damage property of EUV optical elements, which have the same specifications of those in the EUV lithography. And more, it is possible to evaluate the doses for sensitivity of resists.
We evaluate the EUV emission and the spatial distributions of the plasma parameters by use of the two-dimensional (2-D) radiation hydrodynamic simulation in the microplasma high-brightness EUV source. The expected EUV source size, which is attributed to the expanding microplasma by the hydrodynamic motion, was evaluated to be 15 μm at the laser pulse duration of 150 ps [full width at half-maximum (FWHM)]. The numerical simulation suggests that the high brightness EUV source should be produced by use of a dot target based microplasma with the source diameter less than 20 μm. The emission at 13.5 nm was attributed to Sn charge states between Sn<sup>7+</sup> and Sn<sup>12+</sup> with the UTA spectral structure.
The effect of sub-ns duration and sub-mJ energy laser pulse on 13.5 nm extreme ultraviolet (EUV) source diameter and conversion efficiency has been investigated. It was demonstrated that an in-band EUV source diameter as low as 18 μm has been produced due to short scale length of the picoseconds duration laser plasma. Such EUV source is suitable for high brightness and high repetition rate metrology applications.